Head with an air bearing surface having a shallow recessed trailing air flow dam

ABSTRACT

A head has a slider with an air bearing surface having a left trailing air flow dam extending from a trailing pad along a lateral axis, and a right trailing air flow dam extending from the trailing pad along the lateral axis in an opposing direction. A sub-ambient pressure cavity is disposed adjacent to and upstream of at least one of the trailing air flow dams. The trailing pad and the left and right trailing air flow dams together laterally span at least 75% of the width of the slider. Each of the left and right trailing air flow dams is recessed from the primary plane by a step depth in the range 0.05 to 0.5 microns, and the sub-ambient pressure cavity is recessed from the primary plane by a cavity depth in the range 0.8 to 2 microns.

FIELD OF THE INVENTION

The present invention relates generally to the field of information storage devices, and more particularly to air bearing sliders used in such devices.

BACKGROUND

Information storage devices are used to retrieve and/or store data in computers and other consumer electronics devices. A magnetic hard disk drive is an example of an information storage device that includes one or more heads that can both read and write, but other information storage devices also include heads—sometimes including heads that cannot write.

The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board (PCB) attached to a disk drive base of the HDA. Referring now to FIG. 1, the head disk assembly 100 includes at least one disk 102 (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle motor 104 for rotating the disk, and a head stack assembly (HSA) 106. The spindle motor typically includes a rotating hub on which disks mounted and clamped, a magnet attached to the hub, and a stator. Various coils of the stator are selectively energized to form an electromagnetic field that pulls/pushes on the magnet, thereby rotating the hub. Rotation of the spindle motor hub results in rotation of the mounted disks. The printed circuit board assembly includes electronics and firmware for controlling the rotation of the spindle motor and for controlling the position of the HSA, and for providing a data transfer channel between the disk drive and its host. The head stack assembly 106 typically includes an actuator, at least one head gimbal assembly (HGA) 108 that includes a head, and a flex cable assembly 110.

During operation of the disk drive, the actuator must rotate to position the heads adjacent desired information tracks on the disk. The actuator includes a pivot bearing cartridge 112 to facilitate such rotational positioning. One or more actuator arms extend from the actuator body. An actuator coil 114 is supported by the actuator body opposite the actuator arms. The actuator coil is configured to interact with one or more fixed magnets in the HDA, typically a pair, to form a voice coil motor. The printed circuit board assembly provides and controls an electrical current that passes through the actuator coil and results in a torque being applied to the actuator. A crash stop is typically provided to limit rotation of the actuator in a given direction, and a latch is typically provided to prevent rotation of the actuator when the disk dive is not in use.

In a magnetic hard disk drive, the head typically comprises a body called a “slider” that carries a magnetic transducer on its trailing end. The magnetic transducer typically comprises a writer and a read element. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer may be inductive or magnetoresistive. In a magnetic hard disk drive, the transducer is typically supported in very close proximity to the magnetic disk by a hydrodynamic air bearing. As the motor rotates the magnetic disk, the hydrodynamic air bearing is formed between an air bearing surface of the slider of the head, and a surface of the magnetic disk. The thickness of the air bearing at the location of the transducer is commonly referred to as ”flying height.”

Magnetic hard disk drives are not the only type of information storage devices that have utilized air bearing sliders. For example, air bearing sliders have also been used in optical information storage devices to position a mirror and an objective lens for focusing laser light on the surface of disk media that is not necessarily magnetic.

The flying height is a key parameter that affects the performance of an information storage device. Accordingly, the nominal flying height is typically chosen as a careful compromise between each extreme in a classic engineering “trade-off.” If the flying height is too high, the ability of the transducer to write and/or read information to/from the disk surface is degraded. Therefore, reductions in flying height can facilitate desirable increases in the a real density of data stored on a disk surface. However, the air bearing between the slider and the disk surface can not be eliminated entirely because the air bearing serves to reduce friction and wear (between the slider and the disk surface) to an acceptable level. Excessive reduction in the nominal flying height degrades the tribological performance of the disk drive to the point where the disk drive's lifetime and reliability become unacceptable.

Another factor that can adversely affect the tribological performance of the head, and therefore also adversely affect the disk drive's lifetime and reliability, is the extent to which lubricant and other debris are picked up or accumulated on the air bearing surface during operation. Excessive accumulation of lubricant or other debris on the air bearing surface can undesirably change the flying characteristics of the slider, potentially leading to immediate reading or writing errors, head crash, or future tribological failure, and/or redeposit on the disk surface at a location or in a quantity that leads to similar problems.

Accordingly, what is needed in the art is an air bearing design that discourages excessive accumulation of lubricant or other debris on the air bearing surface.

SUMMARY

A novel head is disclosed and claimed. It includes a slider having a leading face and a trailing face, a read transducer adjacent the trailing face, and an air bearing surface. The air bearing surface defines an upstream direction pointing from the trailing face to the leading face and a lateral axis that is orthogonal to the upstream direction. The air bearing surface includes a trailing pad having a major surface adjacent the transducer, with the major surface lying in a primary plane. The air bearing surface also includes a left trailing air flow dam extending from the trailing pad along the lateral axis in a first direction, and a right trailing air flow dam extending from the trailing pad along the lateral axis opposite the first direction. The trailing pad and the left and right trailing air flow dams together laterally span at least 75% of the width of the slider measured along the lateral axis. The air bearing surface also includes a sub-ambient pressure cavity disposed adjacent to and upstream of at least one of the left and right trailing air flow dams. Each of the left and right trailing air flow dams is recessed from the primary plane by a step depth in the range 0.05 to 0.5 microns, and the sub-ambient pressure cavity is recessed from the primary plane by a cavity depth in the range 0.8 to 2 microns.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a contemporary hard disk drive information storage device.

FIG. 2 is an air bearing surface view of a head according to an exemplary embodiment of the present invention (not necessarily to scale).

FIG. 3 is cross-sectional view of the head shown in FIG. 2, taken along the plane of cross-section designated as A-A in FIG. 2. For clarity, only the region of the cross-section near the air bearing surface is shown in FIG. 3 and the step heights are not to scale but rather are exaggerated so as to be easily discernible.

FIG. 4 is an air bearing surface view of a head according to another exemplary embodiment of the present invention (not necessarily to scale).

FIG. 5 is cross-sectional view of the head shown in FIG. 4, taken along the plane of cross-section designated as B-B in FIG. 4. For clarity, only the region of the cross-section near the air bearing surface is shown in FIG. 5 and the step heights are not to scale but rather are exaggerated so as to be easily discernible.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Now referring to FIG. 2, head 200 comprises a transducer 202 for at least reading information from the disk. In certain embodiments, the transducer 202 is a merged thin film magnetic transducer comprising an inductive writer and magneto resistive read element. In such embodiments, the magneto resistive element may be a giant magneto resistive element (GMR) or tunneling magneto resistive element (TMR). In such embodiments, the writer may be a perpendicular magnetic recording (PMR) writer.

Head 200 also comprises a slider 204, which is typically fabricated from a ceramic material such as alumina titanium carbide. Slider 204 includes an air bearing surface 206, which may be formed on the surface of slider 204 by etching or ion milling and has a geometry that may be defined by use of a mask. The slider 204 also includes a leading face 210, and a trailing face 208 adjacent the transducer 202.

In the exemplary embodiment shown in FIG. 2 and FIG. 3, the air bearing surface 206 includes deep cavities 216 and 218, and shallow cavities 220 and 222. During operation, the shallow cavities 220 and 222 can develop a sub-ambient pressure region between the air bearing surface 206 and the surface of an adjacent disk. The sub-ambient pressure may serve to reduce flying height sensitivities to changes in altitude and air bearing geometries.

In the exemplary embodiment shown in FIG. 2 and FIG. 3, the air bearing surface 206 also includes two leading pads 212 and 214 that are adjacent to and upstream of the deep cavities 216 and 218, respectively. The term “upstream” is used herein only to define a directional convention to facilitate description of relative positions on the air bearing surface 206, and does not require the presence or existence of any stream. For example, “upstream” can be understood to refer to a range of directions across the air bearing surface 206 that generally point away from the trailing face 208 and towards the leading face 210. As such, in disk drive applications, upstream directions would ultimately be generally opposite the motion of an adjacent rotating disk surface. An upstream direction would be a direction within the aforementioned range. The term “downstream” is used herein as an antonym of “upstream.”

For each upstream direction, the air bearing surface 206 defines a lateral axis that is perpendicular to that upstream direction. For example, for a zero-skew upstream direction 230 that is parallel to the air bearing surface 206 and parallel to the plane of cross-section AA depicted in FIG. 2, the air bearing surface defines a corresponding lateral axis 232 that is parallel to the leading face 210 or the trailing face 208 (i.e. orthogonal to that upstream direction). The slider width can be measured along lateral axis 232. Of course, non-zero skew upstream directions are also contemplated herein.

The two leading pads 212, 214 may be separated by shallow cavities 220 and 220, respectively, and shallow cavities 220 and 222 may themselves be separated by a longitudinal divider 216.

Now referring additionally to FIG. 3, the leading pads 212 and 214 each include a major surface that is not recessed and instead establishes an air bearing surface datum plane (hereinafter referred to as the primary plane) 300, from which the recession of other surfaces that are parallel to the primary plane 300 may be measured. During operation, the leading pads 212 and 214 can develop a super-ambient pressure region between the air bearing surface 206 and the surface of an adjacent disk, causing the slider to assume a positive pitch attitude. Deep cavities 216 and 218 each include a surface in a plane 330 that is recessed from the primary plane 300 by a deep cavity recession depth 370. The deep cavity recession depth is preferably but not necessarily in the range 3 microns to 4 microns. Shallow cavities 220 and 222 each include a surface in an intermediate plane 320 that lies between the primary plane 300 and the deep cavity plane 330, and that is recessed from the primary plane 300 by a shallow cavity recession depth 360. For example, the shallow cavity recession depth 360 is preferably in the range 0.8 microns to 2 microns.

In the exemplary embodiment shown in FIG. 2 and FIG. 3, the leading pads 212 and 214 are connected by a leading dam 276 that helps prevent particulate contamination from entering the air bearing, and that assists in creating sub-ambient pressure in shallow cavities 220 and 222. The leading pads 212 and 214 also include leading pressurizing steps 224 and 226, respectively. The leading pressurizing steps 224 and 226 each include a surface in a plane 310 that lies between the primary plane 300 and the intermediate plane 320. The plane 310 is recessed from the primary plane 300 by a pressurizing step recession depth 350. During operation, the leading pressurizing steps 224 and 226 can help develop super-ambient pressure between the leading pads 212 and 214, respectively and the surface of an adjacent disk. The pressurizing step recession depth 350 is preferably in the range 0.05 microns to 0.5 microns.

Also in the exemplary embodiment of FIG. 2 and FIG. 3, the air bearing surface 206 includes mid-cavity dams 272 and 274 that are disposed upstream of a central cavity 228. Mid-cavity dam 272 includes a surface in the plane 320, while mid-cavity dam 274 includes a surface in the plane 310. Moreover, mid-cavity dam 272 is oriented to confront the skewed incoming air flow when the head is near the inner diameter of the disk, while mid-cavity dam 274 is oriented to confront the differently-skewed incoming air flow when the head is near the outer diameter of the disk. Because mid-cavity dam 272 is recessed from the primary plane 300 more than mid-cavity dam 274 is, mid-cavity dam 272 tends to allow airflow into central cavity 228 more easily than mid-cavity dam 274 does. This difference in recession can be used by the air bearing designer to desensitize the flying height to changes in the radial position of the head relative to the spinning underlying disk, given that the incoming air flow has a higher velocity near the outer diameter of the disk and a lower velocity near the inner diameter of the disk. If the air bearing designer advantageously opts to use the mid-cavity dams 272 and 274 to desensitize the flying height to changes in the radial position of the head relative to the spinning underlying disk, then the air bearing designer will have more freedom to design downstream features of the air bearing 206, for example to include contamination-reducing features, and/or to reduce air bearing sensitivity to changes in operating altitude, applied bias forces and torques, etc.

In the exemplary embodiment of FIG. 2, the air bearing surface 206 also includes a trailing pad 256 having a major surface adjacent the transducer 202, the major surface lying in the primary plane 300. During operation, the trailing pad 256 can develop a super-ambient pressure region between the air bearing surface 206 and the surface of an adjacent disk that can help maintain a desired flying height at the location of transducer 202. For example, in the embodiment of FIG. 2, the trailing pad 256 creates a region of high pressure, including the highest pressure generated by the air bearing surface during normal operation of the head.

In the embodiment of FIG. 2, a pressurizing step surface 250 is disposed upstream of the trailing pad 256. The pressurizing step surface 250 includes a surface that lies in the plane 310. For example, the step surface may be recessed from the first surface 300 by the pressurizing step recession depth 350 (i.e. 0.05 microns to 0.5 microns). During operation, the pressurizing step surface 250 can enhance the super-ambient pressure between the trailing pad 256 and the surface of an adjacent disk. Such enhanced pressurization may reduce the surface area required for the trailing pad 256.

Trailing pad side portions 246 and 248 can enhance the performance of the pressurizing step surface 250 by partially confining the airflow to pressurize the trailing pad 256. In the embodiment of FIG. 2, the trailing pad 256, and the trailing pad side portions 246 and 248, together form a trailing center pressurizing structure that has the general shape of the letter “W.” For example, the trailing pad 256 includes the center peak of the letter “W,” and the trailing pad side portions 246 and 248 could be considered to be the outer sides of the letter “W.” In the embodiment of FIG. 2, the W that is oriented so that the center peak of the W points in the upstream direction 230.

In the embodiment of FIGS. 2 & 3, the air bearing surface 206 also includes a left trailing air flow dam 260 extending from the trailing pad 256 along the lateral axis 232 in a first direction (i.e. generally to the left from the perspective of FIG. 2), and a right trailing air flow dam 264 extending from the trailing pad 256 along the lateral axis 232 opposite the first direction (i.e. generally to the right from the perspective of FIG. 2). The trailing pad 256 and the left and right trailing air flow dams 260, 264, together laterally span at least 75% of the width of the slider 204. Each of the left and right trailing air flow dams 260, 264 is recessed from the primary plane 300 by the pressurizing step recession depth 350 (i.e. 0.05 to 0.5 microns). Preferably, each of the left and right trailing air flow dams 260, 264 is disposed ahead of a lateral projection of the transducer 202 by a distance measured in the upstream direction 230 that is less than 25 microns.

In the embodiment of FIGS. 2 & 3, the air bearing surface 206 also includes two sub-ambient pressure cavities 252, 254 disposed adjacent to and upstream of the left and right trailing air flow dams 260, 264. The sub-ambient pressure cavities 252, 254 each include a surface in the plane 320 that is recessed from the primary plane 300 by the shallow cavity recession depth 360 (i.e. 0.8 microns to 2 microns). Trailing pad side portions 246 and 248 may also extend around sub-ambient pressure cavities 252 and 254, to assist in the development of sub-ambient pressure within sub-ambient pressure cavities 252 and 254. During operation, the sub-ambient pressure cavities 252 and 254 may develop sub-ambient pressure in much the same way that shallow cavities 220 and 222 do, and thereby shift an effective center of net sub-ambient pressure rearward (towards the trailing face of the slider). Such a shift can facilitate separating the slider from the disk surface dynamically, during operation.

In the embodiment of FIGS. 2 & 3, the left and right trailing air flow dams 260, 264 may serve to reduce back flow from adjacent the trailing face 208 in the upstream direction 230 into the sub-ambient pressure cavities 252, 254, which, in turn, may serve to reduce the accumulation of lubricant and/or other debris in the sub-ambient pressure cavities 252, 254. The left and right trailing air flow dams 260, 264 are recessed from the primary plane 300 because if they were not so recessed they would need to be located too far upstream of the trailing face 208 to acceptably accomplish the back flow reducing function described above. A reason why the left and right trailing air flow dams 260, 264 would have to be located further upstream if they were not recessed from the primary plane 300 is that, during operation, the air bearing 206 typically makes a non-zero roll angle with respect to the disk surface while it is desired that the location of minimum thickness of the air bearing be at or near the location of the transducer 202 (rather than on one of the trailing air flow dams 260, 264).

Region 280 may include a surface that is recessed from the primary plane 300 more than the plane 320 because the two sub-ambient pressure cavities 252 and 254 are disposed over the main body of the slider 204, which typically comprises the ceramic material alumina titanium carbide, while region 282 which includes region 280 comprises overcoat material which is typically alumina. Alumina typically etches away more rapidly than does alumina titanium carbide during fabrication of the sub-ambient pressure cavities 252 and 254 and the region 280.

In the embodiment of FIG. 2, the air bearing surface 206 also includes side pads 242 and 244. The side pads 242 and 244 each include a major surface that lies in the primary plane 300. Accordingly, the side pads 242 and 244 are located further upstream than the aft-most extent of the left and right trailing air flow dams 260, 264. The side pad 242 includes side pressurizing step 262 which includes a surface in plane 310. The side pad 244 includes side pressurizing step 266 which includes a surface in plane 310.

Now referring to FIG. 4, head 400 comprises a transducer 402 for at least reading information from the disk. In certain embodiments, the transducer 402 is a merged transducer comprising both a writer and a read element. Head 400 also comprises a slider 404 that includes an air bearing surface 406. The slider 404 also includes a leading face 410, and a trailing face 408 adjacent the transducer 402.

In the exemplary embodiment shown in FIG. 4 and FIG. 5, the air bearing surface 406 includes deep cavity 428. In the exemplary embodiment shown in FIG. 4 and FIG. 5, the air bearing surface 406 also includes shallow cavity 416. During operation, the shallow cavity 416 can develop a sub-ambient pressure region between the air bearing surface 406 and the surface of an adjacent disk. The sub-ambient pressure may serve to reduce flying height sensitivity to changes in altitude.

In the exemplary embodiment shown in FIG. 4, the air bearing surface 406 also includes two leading pads 412 and 414 that are separated by the shallow cavity 416. Now referring additionally to FIG. 5, the leading pads 412 and 414 each include a major surface that is not recessed and instead lies in the primary plane 300. During operation, the leading pads 412 and 414 can develop a super-ambient pressure region between the air bearing surface 406 and the surface of an adjacent disk, causing the slider to assume a positive pitch attitude. Deep cavity 428 includes a surface in the plane 330 that is recessed from the primary plane 300 by the deep cavity recession depth 370 (i.e. 3 microns to 4 microns). Shallow cavity 416 includes a surface in the intermediate plane 320 that lies between the primary plane 300 and the deep cavity plane 330, and that is recessed from the primary plane 300 by the shallow cavity recession depth 360 (i.e. 0.8 microns to 2 microns).

In the exemplary embodiment shown in FIG. 4 and FIG. 5, the leading pads 412 and 414 also include leading pressurizing steps 424 and 426, respectively. The leading pressurizing steps 424 and 426 each include a surface in the plane 310 that lies between the primary plane 300 and the intermediate plane 320, and that is recessed from the primary plane 300 by the pressurizing step recession depth 350 (i.e. 0.05 microns to 0.5 microns). During operation, the leading pressurizing steps 424 and 426 can help develop super-ambient pressure between the leading pads 412 and 414, respectively and the surface of an adjacent disk.

In the exemplary embodiment of FIG. 4, the air bearing surface 406 also includes a trailing pad 456 having a major surface adjacent the transducer 402, the major surface lying in the primary plane 300. During operation, the trailing pad 456 can develop a super-ambient pressure region between the air bearing surface 406 and the surface of an adjacent disk that can help maintain a desired flying height at the location of transducer 402. For example, in the embodiment of FIG. 4, the trailing pad 456 creates a region of high pressure, including the highest pressure generated by the air bearing surface during normal operation of the head. In the exemplary embodiment of FIG. 4, the air bearing surface 406 also includes trailing pad side portions 446 and 448, which may serve to enhance the pressurization of trailing pad 456.

In the embodiment of FIGS. 4 & 5, the air bearing surface 406 also includes a left trailing air flow dam 460 extending from the trailing pad 456 along the lateral axis 432 in a first direction (i.e. generally to the left from the perspective of FIG. 4), and a right trailing air flow dam 464 extending from the trailing pad 456 along the lateral axis 432 opposite the first direction (i.e. generally to the right from the perspective of FIG. 4). The trailing pad 456 and the left and right trailing air flow dams 460, 464, together laterally span at least 75% of the width of the slider 404. Each of the left and right trailing air flow dams 460, 464 is recessed from the primary plane 300 by the pressurizing step recession depth 350 (i.e. 0.05 to 0.5 microns). Preferably, each of the left and right trailing air flow dams 460, 464 is disposed ahead of a lateral projection of the transducer 402 by a distance measured in the upstream direction 430 that is less than 25 microns.

In the embodiment of FIGS. 4 & 5, the air bearing surface 406 also includes two sub-ambient pressure cavities 452, 454 disposed adjacent to and upstream of the left and right trailing air flow dams 460, 464. The sub-ambient pressure cavities 452, 454 each include a surface in the plane 320 that is recessed from the primary plane 300 by the shallow cavity recession depth 360 (i.e. 0.8 microns to 2 microns). Trailing pad side portions 446 and 448 may also extend around sub-ambient pressure cavities 452 and 454, to assist in the development of sub-ambient pressure within sub-ambient pressure cavities 452 and 454. During operation, the sub-ambient pressure cavities 452 and 454 may develop sub-ambient pressure in much the same way that shallow cavity 416 does, and thereby shift an effective center of net sub-ambient pressure rearward (towards the trailing face of the slider). Such a shift can facilitate separating the slider from the disk surface dynamically, during operation.

In the embodiment of FIGS. 4 & 5, the left and right trailing air flow dams 460, 464 may serve to reduce back flow from adjacent the trailing face 408 in the upstream direction 430 into the sub-ambient pressure cavities 452, 454, which, in turn, may serve to reduce the accumulation of lubricant and/or other debris in the sub-ambient pressure cavities 452, 454. The left and right trailing air flow dams 460, 464 are recessed from the primary plane 300 because if they were not so recessed they would need to be located too far upstream of the trailing face 408 to acceptably accomplish the back flow reducing function described above. A reason why the left and right trailing air flow dams 460, 464 would have to be located further upstream if they were not recessed from the primary plane 300 is that, during operation, the air bearing 406 typically makes a non-zero roll angle with respect to the disk surface while it is desired that the location of minimum thickness of the air bearing be at or near the location of the transducer 402 (rather than on one of the trailing air flow dams 460, 464).

Region 480 may include a surface that is recessed from the primary plane 300 more than the plane 320 because the two sub-ambient pressure cavities 452 and 454 are disposed over the main body of the slider 404, which typically comprises the ceramic material alumina titanium carbide, while region 482 which includes region 480 comprises overcoat material which is typically alumina. Alumina typically etches away more rapidly than does alumina titanium carbide during fabrication of the sub-ambient pressure cavities 452 and 454 and the region 480.

In the embodiment of FIG. 4, the air bearing surface 406 also includes side pads 442 and 444. The side pads 442 and 444 each include a major surface that lies in the primary plane 300. Accordingly, the side pads 442 and 444 are located further upstream than the aft-most extent of the left and right trailing air flow dams 460, 464.

In the foregoing specification, the invention is described with reference to specific exemplary embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. It is contemplated that various features and aspects of the above-described invention may be used individually or jointly and possibly in an environment or application beyond those described herein. The specification and drawings are, accordingly, to be regarded as illustrative and exemplary rather than restrictive. The terms “comprising,” “including,” and “having,” as used herein are intended to be read as open-ended terms. 

1. A head comprising: a slider having a leading face and a trailing face and an air bearing surface, the air bearing surface defining an upstream direction pointing from the trailing face to the leading face and a lateral axis that is orthogonal to the upstream direction, the slider having a width measured along the lateral axis; and a read transducer adjacent the trailing face; wherein the air bearing surface includes: a trailing pad including a major surface adjacent the transducer, the major surface lying in a primary plane; a left trailing air flow dam extending from the trailing pad along the lateral axis in a first direction; a right trailing air flow dam extending from the trailing pad along the lateral axis opposite the first direction, the trailing pad and the left and right trailing air flow dams together laterally spanning at least 75% of the width of the slider; and a sub-ambient pressure cavity disposed adjacent to and upstream of at least one of the left and right trailing air flow dams; wherein each of the left and right trailing air flow dams is recessed from the primary plane by a step depth in the range 0.05 to 0.5 microns, and the sub-ambient pressure cavity is recessed from the primary plane by a cavity depth in the range 0.8 to 2 microns.
 2. The head of claim 1 wherein the trailing pad is shaped like a letter W that is oriented so that the center peak of the W points in the upstream direction.
 3. The head of claim 1 further including a step surface immediately upstream of the trailing pad, the step surface being recessed from the primary plane by the step depth.
 4. The head of claim 1 wherein each of the left and right trailing air flow dams is disposed ahead of a lateral projection of the transducer by a distance measured in the upstream direction that is less than 25 microns.
 5. The head of claim 1 further comprising a deep cavity, the deep cavity being recessed from the primary plane by 3 to 4 microns.
 6. The head of claim 1 wherein the read transducer is part of a merged transducer that also includes a write transducer.
 7. A head comprising: a slider having a leading face and a trailing face and an air bearing surface, the air bearing surface defining an upstream direction pointing from the trailing face to the leading face and a lateral axis that is orthogonal to the upstream direction; and a read transducer adjacent the trailing face; wherein the air bearing surface includes: a trailing pad including a major surface adjacent the transducer, the major surface lying in a primary plane; a sub-ambient pressure cavity recessed from the primary plane by a cavity depth in the range 0.8 to 2 microns; and recessed dam means for reducing back flow from adjacent the trailing face in the upstream direction into the sub-ambient pressure cavity. 